Such filters are used in processes requiring a very good cleaning of gases, for instance gases fed to a detection space of a measuring process, such as gas chromatography. These gases typically pass a plurality of filters of the above-mentioned type. The filters are for instance arranged in series and are connected to a line through which the gas to be cleaned flows. After a passage of time, the filter material is saturated to such an extent that it no longer has a filtering effect. This can be observed, for instance, by means of an indicator which may also be located in the filter material housing. When the filter material of one of the filters is saturated, the filter with the quick-change coupling can be removed from the gas line and replaced by a new filter. The gas-carrying line is provided with a number of coupling pieces to which the filters can be connected by means of the quick-change couplings. When a filter is connected to a coupling piece of the gas-carrying line, the gas flows out of the gas-carrying line via the filter and when no filter is connected to the coupling piece, the connecting openings of the coupling piece are closed hermetically, the gas flows on in the gas-carrying line and no gas is allowed to flow away from the line via the coupling piece or flow from the surroundings into the gas-carrying line. Hence, the quick-change couplings enable a normal continuation of the process for the purpose of which the gases are cleaned, even when a filter is being replaced.
The processes for the purpose of which the gases are cleaned, such as, for instance, detection processes, are becoming increasingly accurate, which necessitates an increasingly better cleaning of the gases. Detection processes with a detection limit of 10.sup.-14, i.e. a detection process in which one particle in 10.sup.-14 particles can be detected, are no longer an exception.
The known filter cannot be used for such highly accurate processes. This is caused by the fact that the known filters are manufactured from synthetic material, such as, for instance, acrylate or lexan. With the known filters, acrylate or lexan is chosen in view of the fact that this material is transparent, which is important for the perceptibility of the indicator and in view of the fact that synthetic material can be processed more easily and was moreover considered to exhibit good resistance to high pressures.
However, the drawback of the synthetic filter material housings and quick-change couplings is that the softeners and similar solvents are released from the synthetic material, particularly when the pressure in the gas-carrying line is low. The released softeners and the solvents disturb the processes for the purpose of which the gases are cleaned. Particularly in the case of accurate detection processes, with a detection limit of 10.sup.-14, for instance, detection becomes impossible.